In most cases ‘dietary supplements’ fall under the category of therapeutic goods in Australia, specifically listed medicines.
The first step, if you are wishing to import dietary supplements into Australia for sale is to confirm whether your products are manufactured in a TGA licenced facility or whether the manufacturing facility has Pharmaceutical GMP certification in a country with whom Australia has a mutual recognition agreement. To do this you need to speak with your manufacture to see if they have a TGA licence or Pharmaceutical GMP.
Please note that GMP for foods is not sufficient for listed medicines it must be pharmaceutical GMP.
Australia does not have a mutual recognition agreement with the USA.